1. Field of the Invention
The present invention relates generally to insulated pipes or conduits and, in particular, to a new and novel insulated pipe construction which avoids deterioration of the insulation due to thermal cycling.
2. Description of the Related Art
In a fluidized bed boiler, there is a need to collect hot particulate matter (solids) and return the solids back to the furnace combustion zone. As shown in FIG. 1, a conduit 10 is used to transport the solids, and is typically comprised of a steel shell 12 lined internally with a multi-layer refractory composite 14. The refractory composite 14 is typically comprised of an insulating refractory layer 16 on the internal diameter (ID) or inner surface 18 of the steel shell 12 and a dense, "erosion resistant" refractory layer 20 covering the insulating refractory layer 16 and which forms a passage 21 defined by an ID 22 of the conduit 10. The hot particulate matter is conveyed within the conduit 10 in contact with the ID 22. The support for the refractory composite 14 is generally by anchors 24 welded to the ID 18 of the steel shell 12, and separate anchors 24 may be provided for each layer of refractory 16, 20.
During normal operation, high temperature solids are conveyed through the conduit 10. The temperature of the solids is in the range of 1500.degree. F.-1800.degree. F., but lower temperatures are possible. At temperature equilibrium, the ID 22 of the dense erosion resistant refractory 20 is at the same temperature as the hot particulate matter conveyed through the conduit 10, while an OD 26 of the steel shell 12 is typically in the range of 165.degree. F.-225.degree. F.
The properties of the refractory 16, 20 vary widely depending on vendor supply, application and process. However, the common properties of this construction are:
1. The insulating refractory 16 is typically cast or gunned onto the ID 18 of the steel pipe 12. However, the greater the need for insulating quality the lower the strength of the insulating refractory layer 16.
2. The dense refractory layer 20 is also typically cast or gunned. This dense refractory layer 20 has significantly higher weight and strength than the insulating refractory layer 16.
The problems with this construction are: (1) Failures of the refractory 16, 20 and (2) damage to the steel shell 12 due to overheating when the internal refractory layers fail. During start-up, the temperature of the conduit 10 increases from ambient to operating temperature. The conduit 10 is heated from the ID 22, since the solids conveyed thereby are the source of the heat. Heating causes the dense refractory 20 to expand radially and since the rate of heating of the insulating refractory 16 and the steel shell 12 lags behind the rate of heating of the dense refractory 20, the dense refractory 20 compresses the insulating refractory 16 causing stresses that exceed the crushing strength of the insulating refractory 16. Additionally, this thermal loading is transmitted to the steel shell 12, causing extremely high hoop stresses therein which could exceed the yield strength of the steel from which it is made. Successive heat-up and cool-down cycles continue the process, tending to crush the insulating refractory 16, leading to overheating and possibly yielding of the steel pipe 12.
The steel shell 12 is generally oriented vertically. When anchors 24 are the only means of vertical support against the force of gravity for the dense refractory 20, the stresses on the anchors 24 are due to bending from the cantilever loading of the refractory 16, 20 thereon. This cantilever loading on the anchors 24 increases as deterioration of the insulating refractory 16 continues. When the anchors 24 bend, the stress in the dense refractory 20 is concentrated at and along each anchor 24. This point/line tension loading on the dense refractory 20 causes cracks. Ultimately, the cracks propagate through the dense refractory 20 and pieces of the dense refractory 20 fall out, which is defined as a failure of the insulated conduit 10.
It has thus become desirable to develop an improved insulated pipe construction which avoids deterioration of the dense refractory due to thermal cycling.